Note: Descriptions are shown in the official language in which they were submitted.
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211PUS04873
HYBRID AIR AND NITROGEN RECYCLE LIQUEFIER
FIELD OF THE INVENTION
The present invention is directed to a process producin~ large
quantities of liquid product via the cryogenic distillation of air.
BACKGROUND OF THE INVENTION
Liquefied atmospheric gases, including nitrogen, oxygen and argon,
are finding increasing uses in industry. Such liquefied atmospheric gases
provide cryogenic capabilities for various industrial processes, are more
economical to transport in merchant supply and provide ready and
economical sources of gaseous product from liquid storage facilities. For
instance, liquid nitrogen is increasingly used to freeze food products, to
cryogenically embrittle used materials for cleaning or recycle, and as a
supply of ~aseous nitro~en inertin~ medium for various industrial
processes.
The conventional process for making large quantities of liquid
nitrogen and/or liquid oxygen from an air feed is to include an expander
scheme with the conventional multiple column distillation system. The
expander scheme provides at least a portion of the large amount of
refri~eration that is required to remove a large percentage of the air
feed as liquid product vis-a-vis a small percentage of the air feed or no
percenta~e of the air feed as liquid product. (As used herein, a ~large
percentage~ of the air feed is defined as at least 15% of the air feed).
This inclusion of an expander scheme with the conventional multiple column
distillation system is generally referred to in the industry as a
liquefier and that is how the term liquefier is used herein.
The most common liquefier probably falls into the category of
nitrogen recycle liquefiers. In a nitrogen recycle liquefier, the
expander scheme is integrated with the recycling of low pressure column
nitrogen overhead such as taught in US Patents 3,605,422 and 4,894,076.
The nitrogen recycle liquefiers, no matter how many expanders there are,
do not try to use the feed air for generating refrigeration before it is
fed into the distillation column systems.
U.S. Patent 4,152,130 introduces the concept of air recycling. In
the air recycle liquefiers, a major fraction of the air streams entering
cold box are compressed to pressures higher than that needed for the
distillation system. At least a portion of the high pressure air is
isentropically expanded to provide the refrigeration needed for
liquefaction while another portion is cooled to a temperature below its
critical temperature, so that liquid air can be obtained upon expansion of
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this cold air stream. This cooled and expanded liquid containing air is
then fed into the distillation system for separation. A portion of the
isentropically expanded, mainly vapor bearin~ air can also be fed into the
distillation system to supplement the vapor feed necessary for the
distillation system. Since all the air, including that fed to the
distillation system, enters the cold box at pressures significantly higher
than that required by the distillation system, the feed air is used for
refrigeration generation or condensation before it enters the distillation
system. As compared to the nitrogen recycle liquefiers, this reduces the
recirculation flow needed for generatin~ the desired refri~eration which
translates into (1) less power loss due to pressure drop, (2) less energy
de~radation due to heat transfer of the recycle streams and (3) less heat
exchanger area. A problem with the air recycle liquefier however, is that
as the liquid demand (as a percentage of feed air) increases, the fraction
of liquid air in the total feed air increases. This will have an adverse
effect on the distillation operation since a large fraction of liquid air
in the feed air means a reduced vapor flow to the distillation system, so
that not enough vapor is rising in the higher pressure column to generate
the boilup for the lower pressure column and to generate the liquid
nitrogen which is demanded as reflux and as product. This problem can be
overcome by vaporizing a portion or all of the liquid air (or some other
liquid process stream) via heat exchange against a condensing stream of
high pressure nitrogen as taught in U.S. Patent 4,705,548. This, however,
introduces an extra step, namely condensation of nitrogen and vaporization
of the liquid air. Since pressure drops as well as energy degradation are
involved in this condensation/vaporization step, it means extra power
consumption as well as an extra heat exchanger for the
condensation/vaporization.
It is an object of the present invention to improve the energy
efficiency of the conventional air recycle liquefier by overcoming the
above described problem.
SUMMARY OF THE INVENTION
The present invention is an improvement to a process producin~ large
quantities of liquid product via the cryogenic distillation if air. In
the process to which the improvement pertains, an air feed is compressed,
expanded to generate refrigeration and subsequently fed to a distillation
column system. The present invention is an improved method to meet the
nitrogen reflux and/or liquid nitrogen product requirements of the process
and comprises:
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(a) compressing at least a portion of the nitrogen overhead from
the distillation column system to a pressure greater than 200 psia, and
more preferably to a pressure greater than nitrogen's critical pressure of
492.9 psia;
(b) cooling the nitrogen from step (a) by indirect heat exchange
against process vapor streams; and
(c) expanding the nitrogen from step (b) wherein said expansion is
performed directly after step (b).
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a conventional process producing
large quantities of liquid product via the cryogenic distillation of air.
Figure 2 is a schematic diagram of one embodiment of the process of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
To better understand the present invention, it is important to
understand the evolution of the air recycle liquefier. The air recycle
liquefier was developed partly in response to the problems of the nitrogen
recycle liquefier relating to the large nitrogen recirculation flow that
the nitrogen recycle liquefier requires for generating the desired
refrigeration. In the air recycle liquefiers, a major fraction of the air
streams entering cold box are compressed to pressures higher than that
needed for the distillation system. At least a portion of the high
pressure air is isentropically expanded to provide the refrigeration
needed for liquefaction while another portion is cooled to a temperature
below its critical temperature, so that liquid air can be obtained upon
expansion of this cold air stream. This cooled and expanded liquid
containing air is then fed into the distillation system for separation. A
portion of the isentropically expanded, mainly vapor bearing air can also
be fed into the distillation system to supplement the vapor feed necessary
for the distillation system. Since all the air, including that fed to the
distillation system, enters the cold box at pressures significantly higher
than that required by the distillation system, the feed air is used for
refrigeration generation or condensation before it enters the distillation
system. As compared to the nitrogen recycle liquefiers, this reduces the
recirculation flow needed for generating the desired refrigeration which
translates into (1) less power loss due to pressure drop, (2) less energy
degradation due to heat transfer of the recycle streams and (3) less heat
exchanger area. A problem with the air recycle liquefier however, is that
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as the liquid demand (as a percentage of feed air) increases, the fraction
of liquid air in the total feed air increases. This will have an adverse
effect on the distillation operation since a large fraction of liquid air
in the feed air means a reduced vapor flow to the distillation system, so
that not enough vapor is rising in the higher pressure column to generate
the boilup for the lower pressure column and to generate the liquid
nitrogen which is demanded as reflux and as product. This problem can be
overcome by vaporizing a portion or all of the liquid air (or some other
liquid process stream) via heat exchange against a condensing stream of
high pressure nitrogen as taught in U.S. Patent 4,705,548. This, however,
introduces an extra step, namely condensation of nitrogen and vaporization
of the liquid air. Since pressure drops as well as energy degradation are
involved in this-condensation/vaporization step, it means extra power
consumption as well as an extra heat exchanger for the
condensation/vaporization.
The present invention is an improved method of meeting the liquid
nitrogen demands which overcomes the above described problem while
retaining the advantages of the air recycle liquefier. The steps of the
present invention comprise:
(a) compressing at least a portion of the nitrogen overhead from
the distillation column to a pressure greater than 200 psia, and more
preferably to a pressure greater than nitrogen's critical pressure of
492.9 psia;
(b) cooling the nitrogen from step (a) by indirect heat exchange
against process vaPor streams; and
(c) expanding the nitrogen from step (b) across a valve or in an
expander wherein said expansion is performed directly after steP (b).
The skilled practitioner will appreciate that the temperature to
which the nitrogen must be cooled in step (b) (hereinafter the 'cooling
temperatureU) is a function of (1) the pressure to which the nitrogen is
compressed in step (a), (2) whether the expansion in step (c) is performed
across a valve or in an expander (ie the isentropic efficiency of the
expansion), (3) the pressure to which the nitrogen is expanded in step (c)
and (4) the desired fraction of the nitrogen which is to be liquid at the
end of step (c). These functionalities are provided on any standard
Mollier chart for nitrogen. The key to the present invention is that the
elevated pressure in step (a) makes it possible to remove significantly
more enthalpy from the nitrogen stream at the cooling temperatures which
can be obtained for the nitrogen stream in the front end/main heat
exchanger ... so much more enthalpy that the enthalpy removing
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condensation step against a vaporizing process liquid stream is no longer
required. In effect, the refrigeration that was formerly indirectly
provided to the nitrogen in the conventional air recycle liquefier (ie by
using the refrigeration to first liquefy a portion of the feed air and
then usin~ this portion of the feed air to liquefy the nitrogen) is now
directlv provided to the nitrogen in the main heat exchanger without an
intervening liquid vaporization step. The increased nitrogen compression
requirement which makes this possible is more than offset by a reduced air
recycle flow through the air compressors since either less or no air is
now required to be liquefied. The present invention essentially provides
the advantages of both the air recycle liquefier (with respect to reducing
the recirculation flow) and the nitrogen recycle liquefier (with respect
to producing some liquid nitrogen directly).
Re~arding the situation where the expansion of the nitrogen in step
(c) of the present invention is performed in a nitrogen expander as
opposed to being performed across a valve, the skilled practitioner will
appreciate that a dense fluid expander is appropriate in this situation
since the feed to the expander is a dense fluid and/or the expander
effluent will have a liquid component. In this situation, the vapor
component of the dense fluid expander effluent can be warmed by indirect
heat exchange against process streams in order to provide additional
refrigeration to the process.
The present invention is best illustrated by applying it to a
conventional air recycle liquefier. Figure 1 is representative of a
conventional liquefier to which the present invention pertains. Figure 1
is based on the teachings of US Patent 4,705,548. Referring now to Figure
1, an ambient air feed in stream 100 is compressed in compressor 110 and
cleaned of impurities which will freeze out at cryogenic temperatures in
cleanin~ bed 310. The resultant stream 201 is combined with an air
recycle stream 234 to form stream 103 which is further compressed in
compressors 140 and 150 prior to being cooled by indirect heat exchange
against warming process streams in heat exchanger 540. A portion of
stream 103 is removed as stream 506 and expanded in expander 152. The
remaining portion of stream 103 is further cooled by indirect heat
exchange a~ainst warming process streams in heat exchanger 541 after which
a second portion of stream 103 is removed as stream 508 and expanded in
expander 153. A portion of expander 153's discharge is removed as stream
124 and warmed by indirect heat exchange against cooling process streams
in heat exchanger 542 after which stream 124 is combined with expander
152's discharge and further warmed by indirect heat exchange against
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cooling process streams in heat exchangers 541 and 540 to form the air
recycle stream 234. The remaining portion of expander 153's discharge is
fed to the bottom of high pressure column 711 as stream 510. The portion
of stream 103 remaining after stream 508 is removed is further cooled by
indirect heat exchange against warming process streams in heat exchanger
542 to form stream 105. A portion of stream 105 is fed to an intermediate
location of high pressure column 711 as stream 106 while the remaining
portion is further cooled by indirect heat exchange against warming
process streams in heat exchangers 552 and 551 before being fed to an
intermediate location of low pressure column 721 as stream 84.
The high pressure column feed streams 106 and 510 are rectified into
a high pressure nitrogen overhead in stream 10 and a high pressure crude
liquid oxygen bottoms in stream 5. Stream 5 is subcooled by indirect heat
exchange against warming process streams in heat exchanger 552, reduced in
pressure and subsequently warmed by indirect heat exchange against a
liquid oxygen product in heat exchanger 550. A portion of stream 5 is
then fed to an intermediate location of low pressure column 721 as stream
910 while the remaining portion is fed to reboiler/condenser 732 at the
top of crude argon column 731 as stream 52.
An argon containing gaseous side stream 89 is removed from a lower
intermediate location of the low pressure column and also fed to crude
argon column 731 in which stream 89 is rectified into an argon-rich vapor
overhead and an argon-lean bottoms liquid in stream 90 which is returned
to the low pressure column. The argon-rich vapor overhead is condensed in
reboiler/condenser 732 against the high pressure crude liquid oxygen
bottoms in stream 52. A portion of the condensed argon-rich vapor
overhead is removed as a liquid argon product in stream 160 while the
remaining portion of the condensed argon-rich vapor overhead is used to
provide reflux for the crude argon column. The portion of the high
pressure crude liquid oxygen bottoms in stream 52 that is vaporized
against the argon-rich vapor overhead is fed to the low pressure column in
stream 15 while
the portion which is not vaporized is fed to the low pressure column in
stream 16.
The low pressure column feed streams 910, 84, 15 and 16 are
distilled into a low pressure nitrogen overhead in stream 130 and a low
pressure liquid oxygen bottoms. The high pressure column and the low
pressure column are thermally linked such that at least a portion of the
high pressure nitrogen overhead in stream 10 is condensed in
reboiler/condenser 722 against vaporizing low pressure liquid oxygen
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bottoms. The condensed high pressure nitrogen overhead is used to provide
reflux for the high pressure column.
The low pressure nitrogen overhead in stream 130 is combined with a
vapor flash stream 85 from flash drum 782 to form stream 131. Stream 131
is warmed by indirect heat exchange a~ainst process streams in heat
exchangers 551, 552, 542, 541 and 540 to form stream 491. A portion of
Stream 491 is removed as a gaseous nitrogen product in stream 488 while
the remaining portion is compressed in compressor 135 to approximately 120
psia to form stream 482. Stream 482 is cooled to near its dew point by
indirect heat exchange against warming process streams in heat exchangers
540, 541 and 542. The resultant stream 163 is subsequently condensed in
reboiler/condenser 723 against vaporizing high pressure crude liquid
oxygen bottoms. The resultant stream 7 is expanded across valve 252 and
subsequently fed as reflux to the high pressure column. A portion of the
low pressure column reflux is removed from the high pressure column in
stream 6. Stream 6 is subcooled by indirect heat exchange against warming
process streams in heat exchanger 551 and flashed in flash drum 782. A
portion of the saturated liquid resulting from this flash is removed as a
liquid nitrogen product in stream 250 while the remaining portion is used
as reflux for the low pressure column in stream 80. The saturated vapor
resulting from this flash in stream 85 is combined with the low pressure
nitrogen overhead in stream 130 to form stream 131.
A nitrogen enriched waste stream 440 is withdrawn from a upper
intermediate location of the low pressure column, warmed by indirect heat
exchange against process streams in heat exchangers 551, 552, 542, 541 and
540 and subsequently removed as a gaseous waste product in stream 479. A
portion of the low pressure liquid oxygen bottoms is removed in stream 117
and subcooled in heat exchanger 550 before being removed as a liquid
oxygen product in stream 70. A portion of the vaporizing low pressure
liquid oxygen bottoms is removed in stream 195 and warmed by indirect heat
exchange against cooling process streams in heat exchangers 542, 541 and
540 before being removed as a gaseous oxygen product in stream 198.
Figure 2 is an embodiment of the present invention as applied to the
flowsheet depicted in Figure 1. Figure 2 is identical to Figure 1
(similar features of Figure 2 utilize common numbering with Figure 1)
except that reboiler/condenser 723 has been eliminated. By elevating the
discharge pressure of compressor 135, it is possible to remove
significantly more enthalpy from stream 482 in the main heat exchanger ...
so much more that the enthalpy removing condensation step in
reboiler/condenser 723 is no longer required. In effect, the
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refrigeration that was formerly indirectly provided to the nitrogen in
reboiler/condenser 723 is now directlv provided to the nitrogen in the
main heat exchanger. Since less air is now required to be liquefied, the
flow of air in stream 105 is decreased. Furthermore, due to increased
liquefaction efficiency, the air compressor recycle flow in stream 124 can
be reduced. Both of these factors more than offset the increased nitrogen
compression requirement in compressor 135.
Although not shown in Figure 2, the shaft work produced from the air
expanders can be used to drive one or more of the compressors in the
process. Similarly, where the expansion of the nitrogen in step (c) of
the present invention is performed in a dense fluid expander as opposed to
being performed across a valve, the shaft work from this dense fluid
expander can be used to drive one or more compressors in the process.
Figure 2 produces almost all of the refrigeration for the process
from expansion of the feed air. It should be pointed out that a recycle
nitrogen stream could be used with at least one additional nitrogen
expander (ie in addition to the dense fluid expander contemplated in step
(c) of the present invention) to supplement the refrigeration. In such a
case, the shaft work produced from this refrigeration providing nitrogen
expander could also be used to drive one or more compressors in the
process.
To further improve the energy efficiency of the process as depicted
in Figure 2, one may increase the operating pressure of the low pressure
column from the conventional range of 17-24 psia to an elevated range
between 25 and 50 psia. This elevated pressure range increases the energy
efficiency of the process by reducing the irreversibility of the
conventional liquefier. Irreversibility is commonly called lost work or
lost exergy. In the distillation system, exergy loss can be reduced by
reducing the driving force for mass transfer. On an x-y equilibrium
diagram, the driving force for mass transfer is shown by the distance
between the equilibrium curve and the operating lines. At the same liquid
to vapor flow ratios in the distillation column, the driving force can be
reduced by elevating the column operating pressure to move the equilibrium
curve closer to the operating lines. This effect is more noticeable in
the low pressure column. Exergy loss can be further reduced in the
conventional liquefier by reducing the driving force for heat transfer in
the front end heat exchanger(s). On a plot of temperature versus enthalpy
change, the driving force for heat transfer is shown by the distance
between the line for the cooling stream and the line for the warming
stream. Elevating the pressure of the low pressure column in turn allows
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elevation of the expander scheme discharge pressure. For a typical inlet
pressure of 600 psia, elevating the expander scheme discharge pressure can
adjust the shape of the cooling curves to allow a smaller average heat
transfer driving force with the same size heat exchanger. An elevated
pressure in the low pressure column also increases the density of the
process gas streams, particularly the low pressure streams. Equipment
sizes can be reduced for capital savings due to the lower volumetric ~as
flows. The upper limit of 50 psia accounts for the fact that, as the
pressure is continually elevated, the benefits of reduced irreversibility
are eventually offset by the prohibitive number of additional trays that
are required in the distillation system. In effect, the elevated pressure
range represents an optimum trade off between reducing the irreversibility
of the process at the expense of increasing the capital requirements of
the process.
It should be noted that where the above described elevated pressure
range is utilized and where a large fraction of the product streams are
not liquefied and are demanded at significantly lower pressure than that
of the low pressure column or vented to the atmosphere, these product
streams can be expanded to provide refrigeration. Such an expansion
exploits the fact that such product stream will also be at an elevated
pressure. Although preferentially they should be isentropically expanded
in an expander, if needed for economical reasons, they could be
isenthalpically expanded across a valve.
In summary, the present invention is an effective method for
increasing the energy efficiency of a conventional air recycle liquefier.
The present invention has been described with reference to a
specific embodiment thereof. This embodiment should not be viewed as a
limitation to the present invention, the scope of which should be
ascertained by the following claims.
RJW\211487:~.APL
